Various types of stress on structural components can cause unnoticed damage therein which propagates progressively and finally results in catastrophic failure. Typically such damage is accompanied by some kind of surface distortion to the material of the structural component. Tensile stress overload causes localized reduction or neck down in cross-sectional area, while impact stress causes surface indentations in the material and compressive stress overloads causes the material to buckle. Furthermore, cyclic stress is known to cause progressive damage in materials but evidence of such damage in homogeneous material such as metal and in laminated composite material such as graphite/epoxy is very difficult to detect. With homogeneous material, fatique cracks develop in planes normal to the outer surface thereof, while ply separation commonly develops in planes parallel to the outer surface of laminated composite materials. Of course, surface distortion also accompanies such damage in either homogeneous or laminated composite materials.
A variety of non-destructive techniques have been employed to detect structural damage in its early stages, such as shadow moire, utrasonics, x-ray, thermographics, laser holography, acoustic emission monitoring and the use of magnetic filings.
However, none of these techniques have been developed for all types of damage encountered and many involve cumbersome equipment. Furthermore, few of these techniques are sufficiently portable for use in field inspections and most have proven unreliable in environments where temperature, humidity, and vibration are encountered.